1. Field of the Invention
The present invention relates to a process for producing a polypropylene film which can readily be impregnated with insulating oils, and hence which can be used in oil-impregnated electrical devices.
2. Description of the Prior Art
Biaxially stretched polypropylene film is known to possess excellent insulating resistance and dielectric characteristics in comparison to insulating paper which heretofore has been used. Consequently, the film has recently been used in the manufacture of various electrical devices such as electrical cables, capacitors and transformers which are required to be compact in size, light in weight and possess high durability. However, the surface of the conventional polypropylene film is very smooth in comparison to the surface smoothness of the conventional insulating paper, so that when it is used in a convoluted form, the surfaces of the film or the surface of the film and the surface of the electrode foil tend to stick and to adhere tightly to each other. Consequently, it is difficult to sufficiently eliminate air or water which is entrained in the central portion of the convoluted material even when it is dried in a vacuum. Further, when the convoluted material after vacuum drying is impregnated with an insulating oil, the portions of the material unimpregnated with the insulating oil form locally within the convolution because the impregnability of the material is inferior. If such a convoluted material is subjected to a voltage, it suffers the drawback that a corona discharge is liable to occur with the result that premature dielectric breakdown undesirably occurs under low voltage conditions in the unimpregnated portions.
In order to reduce these drawbacks, a method has been proposed in which a polypropylene film is used in conjunction with insulating paper as a dielectric, i.e., as a composite dielectric, in the fabrication of film capacitors. However, the dissipation factor of the insulating paper is large and the dielectric strength thereof is low. Moreover, it is very difficult to thoroughly eliminate water which is entrained so that the use of such insulating paper makes it difficult to obtain a capacitor having satisfactory characteristics. Still another method which improves the impregnating ability of insulating oil involves roughening one or both surfaces of the polypropylene film by an embossing treatment which forms irregularities on the surface of the film. But in such a method, a complicated film manufacturing process is required and in order to achieve a greater degree of oil impregnability of the film, it is necessary to increase the depth of the surface irregularities of the film. Furthermore, the characteristics of the film tend to decrease and the relationship of the oil impregnability of the film to the concavity of the film surface inversely decreases. A need therefore, continues to exist for a polypropylene film which is useful in the manufacture of electrical devices which possess superior dielectric characteristics, a susceptibility to being impregnated by insulating oils and which exhibits none of the disadvantages described above.